Optical information systems have proven to be very useful in the design of two-dimensional (2D) pattern recognition systems. Recently, interest in three-dimensional (3D) optical information systems has increased because of its vast potential in applications such as object recognition, image encryption as well as 3D display. Digital holography is attractive for visualization and acquisition of 3D information for these various applications. In digital holography, 3D complex (magnitude and phase) information can be reconstructed at arbitrary depths and perspectives. The 3D complex information might provide more discriminant features for the recognition of microorganisms.
Identification and recognition of microorganisms is desirable for several applications. It may be used to diagnose an infection caused by specific bacteria or detect biological weapons for security and defense, for example. It may also be used for monitoring plankton in the ocean. Quantification of microorganisms may be useful information for wastewater treatment facilities.
Several challenges accompany the automatic recognition of living organisms, such as; they are not rigid objects, they vary in size and shape, and they can move, grow, and reproduce themselves depending on growth conditions. Bacteria and algae, in particular, have relatively simple morphological traits, however they are very tiny and there exist many variants among the same species. Additionally, they may occur as a single cell or form an association of various complexities according to environmental conditions.
One method that can be considered for 3D complex information with for identification and recognition of microorganisms is off-axis digital holography since it requires only a single exposure in separating the original image from the undesired DC and conjugate images. However, off-axis digital holography has a number of drawbacks. Only a fraction of the space-bandwidth product of the photo sensor is used to reconstruct the 3D image, which results in substantially reduced quality of visualization and compromises resolution. As a result, it reduces the accuracy of object recognition. In addition, the angle between the object beam and reference beam during the holographic synthesis is a function of the reconstructed image size, which creates problems in monitoring dynamic scenes containing living objects.
Another identification and recognition method that has undergone study in an attempt to overcome these problems is phase-shifting on-line digital holography. This technique requires multiple interferogram recordings with phase shifts in the reference beam. The multiple exposures are used to remove the DC and the conjugate images in the interferogram and the Fresnel diffraction field of the 3D object is obtained. However, this procedure is not suitable for dynamic events such as moving 3D microorganism and is sensitive to external noise factors such as environmental vibration and fluctuation.
Accordingly, there is a need in the art to overcome the aforementioned problems of existing techniques for microorganism identification and recognition.